[engineer]

it is not a fluid problem. A change now to glycol fluid requires a complete change in all the parts having rubber seals. not worth it. Change the master and the rears.

[Scarebird]

1977-78 Seville 2.75"
1979 Seville, 1979-81 WS6 2.5"
1979-85 Eldorado, 1980-85 Seville 2.125"

[scottk]

Scarebird, are you saying the listings for the 77-8 Seville calipers as being 2.5" are incorrect? They looked like 2.5s to me the last time I saw one. The listings say 2.5" for the caliper bores, pistons, and rebuild kits, and the online pictures appear to agree. Do you have one there to measure?

[Scarebird]

yes, we saw a set last week at a local boneyard- build date was 5/78. Easiest way to tell is the pin spacing; 77-78 have 7", while the 79's are 5-1/2". NAPA book confirms as 2-15/16" bore. My thought is that Caddy used them on the 77-78 El Dorado also, and GM said uh-uh to smaller size till Pontiac came out with the WS6 to share with- 2-15/16 would be overkill (and much more prone to rear lockup).

[critter]

So, if I used the 78 Seville calipers, I still would have the problem of the larger piston area which is already killing me?

I need a small piston caliper and the E-brake capability would be a plus. So, the way I see it, I have two choices right now:

1 - Find a small piston caliper without an ebrake that will mount on the 7 inch Seville backing plates I already have. (Only thing I see out there are the Wilwood GM III calipers with the 2.38 bore)

2 - Find brackets to mount the existing WS6 style calipers I already own.

I've already used the calipers and rotors I own using the other backing plates. I hate these backing plates because they don't seem very sturdy. I've already stripped out one brake pin hole and using a helicoil isn't working because I can't get it in straight. I could easily skip the Ebrake for now. That's making the Wilwood's very appealing.

[critter]

I called the local parts house today. They say the 78 Seville calipers are 2.5 inch. Decisions, decisions.

[scottk]

Since your parts dealer says they are 2.5", you should pick up a set and measure them. If they're right, use them. If not, return them.
The pics above are the 7" spread rears, and the piston looks a lot smaller than the front, which is 2 15/16". They both take the same pad design. When I look it up at NAPA, it is 2.5".

I still cant say it will solve all your problems, but it would be a start down the right road.

[critter]

Heck no. Been too danged busy at work. All I managed to do was make the phone call. I might get a chance to actually look at one tomorrow if it slows down some.

[scottk]

Oh, you can buy an oversized pin for the stripped out bracket instead of Heli-coil. NAPA sells them. I think they're self tapping.

[critter]

Now he tells me!

I actually located a set of 79 TA backing plates today. They should be shipped tomorrow so I now intend to use the other 79 TA parts (rotors, calipers, ebrake cables, MC and combination valve). I'm also going to swith the front calipers back to the stocker single pistons until I can gather the resources and do the 1LE upgrade there. When I do I'll probably use the Wilwood GMIII caliper.

I want to thank you all for the input. I've learned a great deal.

[critter]

I located the proper backing plates to install the WS6 calipers and Hawk pads that I was using with the cheap, Ebay mounting plates before. There is still WAY too much pedal travel for my liking. I now understand that this is because I'm trying to push too great of a volume of fluid based on the square inch need of the pistons on all four corners. I'm already using a master cylinder with the 1 1/8 inch bore so a bigger MC isn't much of an option. The only way I know to get out of this is to reduce the front piston size by switching calipers up there. If I go that direction I'll switch to the B-body spindles and do the 1LE big brake upgrade.

The brakes do stop the car very well but you better be a big ole boy to lock them up.

[scottk]

did you check the pushrod hole depth in that ws6 master cyl to be sure it is compatible with your booster?
the 79 4wldisc unit might have had a deep hole like the older chevys and corvettes.

1 1/8" should be fine for what you're running now if you swapped to the 2.5" rears. the fronts you said you are using are very close to stock area.

BTW, i found out i was wrong about the ws6 booster being a straight mount, but pushrod compatibility is something that should always be checked.

[critter]

Hmmm. So the rod from the pedal to the booster might not be long enough? Sorry, I'm not visualizing what you're saying.

No, I haven't looked at that.

[scottk]

rod between booster and master

[critter]

Gotcha. That's an easy check. I'll try it tomorrow night. Thanks!

[scottk]

i didn't think any masters after 1971 had the deep hole except the corvette, but you should still check.

[critter]

I'm pretty sure that mine was like the one on the right but I'd rather be sure. I'll check it out.

[Scarebird]

Well, we ran in to a 78 Seville at the yard, this one has a 2.5" piston (measured) and the 7" bolt spacing.

[Skip Fix]

I could swear when I did my conversion to 81 booster and master there was an adjustable rod from Ponmtiac used, been too many years to remember.

[critter]

My rod seems to make positive contact but the pedal travel is quite a bit. I've adjusted the rear discs and Ebrake now. It is better but still not what I wanted. I've not driven it much due to the weather conditions here. I'm hoping it tightens up a some as it seats and wears in.